1. Field of the invention
This invention relates to a method of making a squirrel-cage rotor for ac motors wherein a squirrel-cage conductor includes bar conductors embedded in slots of a rotor core and two end rings connecting both ends of the slot-embedded conductors respectively.
2. Description of the prior art
In a conventional method of making squirrel-cage rotors for ac motors, silicon steel plates are laminated for composition of a rotor core. The rotor core is then provided with a squirrel-cage conductor by means of die casting with use of molten aluminum. More specifically, the rotor core is accommodated in a metal die. Molten aluminum is poured into the die so that bar conductors embedded in a plurality of slots formed in the outer periphery of the rotor core and two end rings disposed at opposite ends of the rotor core are integrally formed.
In the above-described method, however, the temperatures of a melting furnace for aluminum and the die casting metal die are raised since the squirrel-cage conductor including the bar conductors and the end rings is formed by means of die casting. Consequently, the atmospheric temperature in a work section in a factory or the like is increased. This poses a problem. Furthermore, since working personnel needs to wait for the temperature of the melting furnace to be sufficiently raised up, the working efficiency is lowered. Furthermore, the above-described method requires a step of cutting a pouring gate for the molten aluminum and a step of cooling the rotor core. Thus, the number of steps is increased in the manufacture of the rotors. Additionally, since it is difficult to stabilize fluidity of the molten aluminum, blowholes or cavities are sometimes produced in the bar conductors and the end rings. Consequently, when the quality of products is lowered, the rotor resistance, balance in rotation and the like are varied in the ac motors, which reduces the characteristics of the motors.